Laser has been an invention that human beings fell proud and it has characteristics such as accuracy, quickness and convenience, easy use and strong anti-interference performance. Laser technologies as developed from laser have solved many technical problems that traditional technologies cannot solve.
A laser range finder produced by integrating laser technologies with electronic technologies increasingly draws attention of industries such as civil use, military use and industrial concerning aspects such as length, height, distance, speed and shape measurement and has already been applied extensively to the following fields abroad: large industrial and mining enterprises, electrical power and petrochemical industry, water conservancy, communications, environment, architecture, geology, police affairs, fire fighting, explosion, navigation, railway, anti-terrorism/military affairs, scientific research institutions, agriculture, forestry, real estate, recreation/outdoor sports and so on.
A laser distance-measuring device based on the principle of phase difference detection uses a modulated laser beam to irradiate a detected object, and the laser beam is bent back by the detected object, and the phase difference occurring in the round trip of the laser beam be converted into a distance from the detected object, which is applied to short-distance measurement with high precision, and the accuracy and the precision of measurement by means of the said laser distance-measuring device be affected by properties of the inner parts of the said device. The higher the precision of the laser distance-measuring instrument is, the more complicated its circuit is and the more precise apparatuses are needed. Therefore, it cannot be neglected that a phase shift be generated by the instrument, due to the impact on the performance of the apparatus exerted by environmental factors, such as temperature and apparatus lifetime. The prior art mostly uses the theory of phase difference compensation between the inner and the outer optical path to eliminate additional phase shift from a circuit system, to ensure that data measurement is not affected by ambient environmental factors. The phase difference compensation theory to eliminate additional phase shift described in the present invention is summarized as follows:
Assuming that the phase differences in retard resulting from a distance-measuring signal travelling through an inner optical path and an outer optical path sequentially be Ψin and Ψout, the phase detection results in a phase detector for dual-wavelength optical wave transmitted by a laser transmitting device passing through a first receiving device be Ψin1 and Ψout1, the phases of the said dual-wavelength optical wave received by a second receiving device be Ψin2 and Ψout2 respectively, an additional phase shift generated by an electronic signal generated by a signal generating device in the instrument in a first portion of signals during circuit transfer and photoelectric conversion be ΔΨ1, an additional phase shift generated by an electronic signal generated by a signal generating device in the instrument in a second portion of signals during circuit transfer and photoelectric conversion be ΔΨ2, and a signal phase generated by the transmitting device be Ψtransmit, transmit the phase comparison result of the inner and outer optical path distance-measuring signal ein and eout in the phase detector is as follows:Ψin2=ΔΨ2+Ψin+Ψtransmit Ψout2=ΔΨ2+Ψout+Ψtransmit Ψin1=ΔΨ1+Ψtransmit Ψout1=ΔΨ1+Ψtransmit 
In the above formulas, the change of operating status of Ψin1, Ψout1 ΔΨ2 and ΔΨ2 accompanied with the instrument under different environment be the random phase shift, and cannot be solved by precise calculation. Therefore, in a short period of time in the distance measurement by using an inner and outer optical path alternately, it can be deemed that there is no additional phase shift occurred during the inner and outer optical path alternate with each other. The difference value of the phase comparison results between the inner and outer optical path be taken as a measurement result accordingly, i.e.
  ϕ  =                    ϕ        out            -              ϕ                  i          ⁢                                          ⁢          n                      =                            (                                    ψ                              out                ⁢                                                                  ⁢                2                                      -                          ψ                              out                ⁢                                                                  ⁢                1                                              )                -                  (                                    ψ                              i                ⁢                                                                  ⁢                n                ⁢                                                                  ⁢                2                                      -                          ψ              ⁢                                                          ⁢                              i                                  n                  ⁢                                                                          ⁢                  1                                                              )                    =                                    (                                          Δ                ⁢                                                                  ⁢                                  ψ                  2                                            -                              Δ                ⁢                                                                  ⁢                                  ψ                  1                                            +                              ψ                out                                      )                    -                      (                                          Δ                ⁢                                                                  ⁢                                  ψ                  2                                            -                              Δ                ⁢                                                                  ⁢                                  ψ                  1                                            +                              ψ                                  i                  ⁢                                                                          ⁢                  n                                                      )                          =                              ψ            out                    -                      ψ                          i              ⁢                                                          ⁢              n                                          
The above result Φ has already eliminated the impact exerted by instable additional phase shift, and it ensures the precision of distance measurement. There be the below calibration methods in the prior art:
(1) A single-transmitting and single-receiving system, namely, an optical signal transmitted via a single optical path and an optical signal received via a single optical path, and the inner and outer optical path be switched over by a controllable mechanical device, and phase calibration is performed by calculating phase values of the inner and outer optical path when switching over to eliminate an uncertain phase interference caused by the environment. Because of a mechanical switch, mechanical response time is long (generally an order of magnitude of several hundred milliseconds) so that real-time calibration cannot be performed, and furthermore the mechanical structure is relatively complicated which might easily come to mechanical wear and malfunction, and lead to a shortened lifetime. As a result, it is not suitable for working as an industrial precision instrument.
(2) A single-transmitting and dual-receiving system, namely, a light beam is transmitted via a signal optical path, and the inner optical path signal and the outer optical path signal are received respectively via the dual paths. The two signals received via the dual paths are processed respectively and are calculated for phase difference to eliminate an uncertain phase interference caused by the environment accordingly. The system uses two avalanche photo diodes (avalanche photo diode, APD) to receive the inner and the outer optical path signal respectively. Such dual-path amplifying circuit is apt to generate same-frequency interference. Furthermore, since avalanche voltage of each APD is different, the resultant phase shift is different. The avalanche voltage difference of APD needs to be limited less than 1V to ensure that the phase shift of different APDs are close. Therefore the requirement for APD is relative rigid and its production procedure is also increased.
(3) A conventional dual-transmitting and single-receiving system, namely, two light beams with the same wavelength are transmitted independently via dual optical paths, and the inner optical path signal and the outer optical path signal are sequentially received respectively by a receiving device. The two signals received via dual paths are processed respectively and are calculated for phase difference to eliminate an uncertain phase interference caused by the environment accordingly. The system generates two paths of optical wave signal with the same wavelength respectively by using two independent photoelectricity generating devices. Because two photoelectricity generating devices, especially two laser tubes, have the different working time of the inner and the outer optical path in operation, and because the above said principle cannot be used for eliminating different temperature drift extremely probably caused by the difference performance of the two lasers, a drift of the measured distance exist.
It is concluded that all of the above mentioned three solutions have drawbacks in practical application.